I attend an FSSP parish. On Sunday, I wear my “Sunday Best” - nice dress shoes, nice skirt, and professional top, and makeup. On Saturdays, when I help with the altar flowers, I wear a more casual shirt, a modest but casual skirt (kakhi skirt, etc.), and comfy shoes like sneakers or sandals. Most of the time on Saturdays I wear light or no makeup. On the weekdays, when I go to daily mass, I wear what I wear to work, which is “Business Professional” - dress slacks, pressed jackets and blouses, etc. But I also wear jeans sometimes on the weekdays if it’s a Jeans Day at work, like I did today.
**I think the most important element is not so much slacks vs skirts, it’s modesty. **You don’t want to be a distraction to other parishioners or an occasion of sin by wearing something tight, or sheer, or low-cut. You don’t need to wear anything “dressy” or expensive, simply modest and lady-like. But you also don’t have to dress like a nun. Yes, I wear jeans to weekday masses on occasion like I did this morning, but they’re not skin tight. If my top is a bit low-cut, I’ll wear a shell underneath it so as not to flash the priest and altar boys at the rail during communion.

My skirts all hit me about mid-calf or lower so that I can easily get up and down from the altar rail and kneel during mass without issues. I know a lady that will use a lacy scarf for a head covering that she can drape around her neck if she’s afraid that her blouse may adjust and dip down a bit.
As far as modesty goes, this is what my cassock-wearing priest had to say about it at the beginning of last summer: “Don’t complain to the priests about modesty and the Texas Heat. Our necklines are higher, our sleeves are longer, our hems go all the way to the ground, and we’re wearing all black.”